Gene expression in two hybrid cell types constructed by nuclear transplantation was examined. In the first type of hybrid cell, dormant erythrocyte nuclei obtained from mature avian red cells were reactivated by transplantation into enucleated mouse or chick fibroblasts. When transplanted into S-phase fibroblasts, nuclei were not only reactivated but also entered the S-phase of the cell cycle; at least half of successfully transplanted nuclei completed replication of the entire genome. Furthermore, the cytoplasmic factors required for entry into S-phase could be transmitted from cell to cell using techniques of microinjection. Synthesis of three forms of globin--two alpha chains and one beta chain--was also detected in the hybrid cells. Of particular interest, it was established that the beta-family chain was a species normally made only during very early embryonic development; thus, the nuclei were, in effect, at least partially re-programmed with respect to globin gene expression. Using cloned globin gene sequences as molecular probes, it was determined that nearly 15% of the stable RNA made by the cells was complementary to this early embryonic rho-globin gene. Correlations of rho-gene activation with structural changes in the gene will be established. In the second system, activation of the gene-encoding liver-specific, steroid-inducible tyrosine aminotransferase (TAT) was observed in mouse fibroblast nuclei after transplantation into enucleated rat hepatoma (HTC) cells. Molecular analysis of this phenomena using cloned TAT gene sequences is in progress. Techniques for large-scale production and definitive identification of these cells, for electrophoretic resolution of mouse and rat TAT enzymes, and for quantitation of TAT mRNA levels have already been established. The system is now ripe for experiments designed to determine the molecular nature of the putative cytoplasmic transacting elements responsible for activation of the TAT gene. (G)